The large-scale implementation of photovoltaic (PV) energy production could benefit from the use of non-toxic, earth-abundant materials that lend themselves to high-volume manufacturing processes. Sulfides based on Cu, Zn, and Sn provide interesting possibilities as inexpensive thin-film PV absorber materials, since their optical properties (band gaps, carrier mobilities, absorption coefficients) can be tuned by forming ternary and quaternary sulfide compounds. For example, the quaternary sulfide Cu2ZnSnS4 (CZTS) or selenide Cu2ZnSnSe4 (CZTSe) or sulfide-selenide Cu2ZnSn(S,Se)4 (CZTSSe) are relatively new, thin-film solar cell absorber materials with a theoretical single-junction energy conversion efficiency limit of about 30% due to their optimal band gaps (˜1.5 eV) and a high absorption coefficients (>104 cm−1). Also, the raw materials needed to fabricate CZTS or CZTSSe are relatively low-cost and produced in the US, in contrast to those used for current commercial non-silicon, thin film PV modules (e.g., Te in CdTe and In and Ga in CIGS). Despite the very high theoretical efficiency (over 30%) of CZTS or CZTSe, the best efficiencies achieved in the laboratory to date have not exceeded ˜11% (CZTS˜8.5%, CZTSe˜9.15%, CZTSSe˜11.1%). The binary sulfides, e.g., Cu2S and SnS, are also of interest as even cheaper PV absorber materials with theoretical efficiencies near 25%.
A major problem in sulfide PV materials is that high temperature annealing treatments are often employed to obtain large grain sizes in order to minimize recombination losses at grain boundaries, and these treatments may lead to vapor-phase loss of volatile species (e.g., S and Sn in CZTS) and thus stoichiometry changes and phase separation.
A challenge in the development of PV thin film compounds based on sulfides is that the ideal stoichiometry suitable for the best PV properties may not be known and further cannot be controlled uniformly across a large-area thin film. In stoichiometric materials, minor deviations from stoichiometry can easily result in very large changes in the thermodynamic chemical potentials of the components and hence the physical/optical properties of the material.